HP OpenVMS Systems Documentation
DECnet-Plus for OpenVMS
The Transport layer ensures the reliable transfer of data. Each Transport protocol provides for the establishment of a transport connection, which carries a stream of two-way communications traffic between two processes on the same or different systems. A transport connection is a temporary logical connection that normally exists until one of the processes terminates the connection.
The Transport layer offers an interface to the service user at the
Session layer. DNA applications can use either the $IPC or $QIO
programming interface to make a connection request through the
proprietary Session Control layer to the Transport layer. OSI
applications can make connection requests through ACSE, the
Presentation layer, and the Session layer to the Transport layer.
3.6.1 Transport Layer Ports and Session Layer Ports
DECnet-Plus uses a mechanism called logical link for communication between processes. A Phase IV logical link in DECnet-Plus terms is either a Transport layer port or a Session layer port.
A logical link is a temporary connection either between processes on
source and destination systems or between two processes on the same
system. A logical link carries a stream of regular data and interrupt
data of full-duplex traffic between two user-level processes. Each
logical link is a temporary data path that exists until one of the two
processes terminates the connection.
3.6.2 Supported Transport Protocols
The OSI Transport Protocol, the implementation of which provides the OSI Transport Service, bridges the gap between the service provided by the network and the service desired by the transport user.
The Transport layer supports both the DIGITAL NSP and the OSI Transport
Protocols. An OSI application makes a connection request to the
Transport layer to access the OSI transport. The choice of protocol for
a non-OSI connection is made during connection establishment: either
the connect request specifies a Transport protocol, or the Session
layer selects an appropriate Transport Protocol. Multiple transport
connections, using any of the Transport Protocols, can be made
22.214.171.124 OSI Transport Protocol
The OSI Transport Protocol permits communication between DECnet-Plus systems and other vendors' systems that also implement the OSI Transport Protocol.
This protocol can use two types of ISO network service:
The OSI transport conforms to the RFC 1006 Standard and to the RFC 1006 extension Internet Draft. They define how to implement ISO 8073 Transport Class 0 on top of TCP (RFC 1006) and how to implement ISO 8073 Transport Class 2, non-use of Explicit Flow Control on top of TCP (RFC 1006 extension). The network service used is provided by TCP.
Table 3-3 describes these classes, their functions, and which network service can be used.
|Protocol Class||Functions||Network Service|
|TP 0||Provides a basic transport service.||CONS and RFC 1006|
|TP 2||Provides all functions of TP 0 plus multiplexing of more than one transport connection over a network connection or TCP connection. Also provides flow control over CONS.||CONS and RFC 1006 extension|
|TP 4||Provides all functions of TP 2 plus error detection and recovery.||CONS and CLNS|
Some other differences are that:
DIGITAL recommends TP 4 for both DNA and OSI applications because it has the greatest set of capabilities suitable for all purposes, unless when using RFC 1006 or RFC 1006 extension.
When a transport user sets up a transport connection, a preferred
protocol class for the connection is specified in the connection
request. The responding transport user must either agree to this
protocol class, or suggest an alternative protocol class that is
acceptable to the initiating user. If no such agreement is possible,
the transport connection cannot be set up.
126.96.36.199 Network Services Protocol (NSP)
The proprietary NSP provides a transport service that supports error detection and recovery and uses CLNS. NSP capabilities are similar to those of OSI Transport Protocol class 4 for CLNS connections.
NSP makes a transport connection between two DECnet-Plus systems or between a DECnet-Plus system and a Phase IV system. NSP is the only transport that can connect to Phase IV systems.
NSP establishes transport connections by exchanging control messages
with a peer NSP module. The connection comprises two data subchannels,
one for normal data exchange and one for other data (such as expedited
data messages and flow control messages). NSP transfers data in
segments, if necessary. The segments are then reassembled in correct
sequence at the destination.
3.6.3 OSI Transport Service
The Transport layer provides the OSI transport service, which fulfills communications requirements for clients in the higher layers. The OSI transport service software implements the OSI Transport Protocol, allowing two OSI transport users to set up and use an OSI transport connection. A transport user uses the OSI transport service to set up a transport connection with another transport user.
The OSI transport service is a consistent interface to transport users, regardless of the type of network over which they make OSI transport connections.
DECnet-Plus for OpenVMS offers three classes of OSI transport service,
each of which is appropriate to a particular network service because
each provides the additional functions not provided by that service. TP
0, 2, and 4 identify the different classes of OSI transport service
offered in DECnet-Plus for OpenVMS (see Table 3-3).
188.8.131.52 OSI Transport Service Functions
Transport users employ the OSI transport service to set up connections and exchange data. Depending on the type of network service used by an OSI transport connection, the OSI transport service offers all or only some of the following functions:
When a transport user makes a connection request, one of the parameters of this request is the name of a transport template, which is a predefined set of parameters for setting up the OSI transport connection. An OSI transport template specifies the type of network service to be used for the OSI transport connection, and the preferred protocol class.
You can use an OSI transport template to provide defaults for any
parameters not included in the connection request.
184.108.40.206 OSI Transport Connections
The Network layer permits communications between network entities in open systems on a subnetwork, intermediate systems, or both. The Network layer provides conformance to ISO standards for packet formats and network addressing:
An OSI transport connection between two OSI transport users is supported by a network connection between the end systems on which the OSI transport users are running. This network connection is set up and maintained by the Network layers of the two end systems. The Network layer, therefore, provides a network service to the OSI transport service.
Service-type conversion takes place only in the Transport and Network layers. This means, for example, that a presentation connection must be supported by a session connection, but that a change to using the connectionless network service can be done through the functions of the Transport layer.
The type of network service used depends on the topology of the network
between the two end systems. The transport user selects the type of
network service for a transport connection by supplying in the
connection request a transport template that specifies the desired
220.127.116.11 Connectionless-Mode Network Service (CLNS)
Connectionless-Mode Network Service is provided to OSI transport and operates according to a datagram model. Each message is routed and delivered to its destination independently of others. For example, the DNA Network (Routing) layer provides this type of service. The Network layer allows a CLNS connection using an X.25 virtual circuit in one of two ways:
CLNS offers the following features:
With CLNS, the Network layer supports three types of connections:
Both forms of CLNS support only TP 4 connections.
18.104.22.168 Connection-Oriented Network Service (CONS)
Connection-Oriented Network Service is provided to OSI transport and operates according to a connection-oriented model. A connection is set up between two communicating end users, is used for data exchange, and is then broken by either end. SDUs sent over the connection do not have to contain a destination address. For example, X.25 provides this type of service.
CONS offers the following features:
Note that in each of these cases, the two end systems are on the same
3.7.2 Broadcast Network Connections: The Routing Process
With CLNS, the Network layer receives user data from the Transport layer and determines the path along which the data travels to its destination. This decision process is the main function of routing. The Network layer provides end-to-end data transfer, or routing, as a service to clients in the Transport layer. This data transfer between the two communicating systems is called a network connection.
An end system (ES) is either the source or destination of data sent over a network connection. An end system receives data units, called packets, addressed to it and sends data units to other systems on the same subnetwork. (DECnet Phase IV calls this type of system an "end node" and the "subnetwork" an area.)
An intermediate system (IS) is a routing system that receives data packets from a source end system, or from the previous intermediate system on the route, and passes them on to the destination end system, or to the next intermediate system on the route. (DECnet Phase IV calls this type of system a "router.")
An intermediate system interconnects two subnetworks. A subnetwork is a network, within a group of interconnected networks, of OSI systems that all use a common addressing format. A subnetwork forms an autonomous whole, for example: ISO 8802-3 (CSMA-CD) LAN, HDLC data link, or X.25 connections.
In the Network layer, the ES-IS protocol (ISO 9542) provides the communication between an end system and the nearest intermediate system. ES-IS is a connectionless protocol. IS-IS protocol (ISO 10589) provides the communication between two intermediate systems. The Internet standard provides the end-to-end connectionless link between end systems. Figure 3-6 shows how ES-IS relates to other ISO protocols.
Figure 3-6 ES-IS and IS-IS Protocols